1. Field of the Invention
The present invention relates to a storage system capable of generational management, and a back-up method for a storage system.
2. Description of the Related Art
For example, in various organizations, such as businesses, self-governing bodies, schools, or the like, large volumes of data of various different types are managed. These large volumes of data are managed by a storage system formed separately from a host computer. The storage system is constituted by comprising at least one or more storage device, such as a disk array device, for example.
A disk array device is constituted by arranging storage devices, such as hard disk drives, or semiconductor memory devices, for example, in an array configuration. The disk array device provides a storage region based on a RAID (Redundant Array of Independent Inexpensive Disks).
In the storage system, by managing the storage contents of a main volume, separately from the main volume, provision is made for cases in which data in the main volume is lost, or the like. For example, as one such method, a technique is known whereby the contents of the volume at a prescribed point in time are saved, by copying the storage contents of the main volume, exactly, to a secondary volume (Japanese Patent Laid-open No. (Hei)11-259348). In another such method, a technique is also known whereby an image (snapshot), which takes a static impression of the main volume at a prescribed point in time, is acquired, in such a manner that the data of the main volume can be managed in a plurality of generations (Japanese Patent Laid-open No. 2002-278819).
If the storage contents of the main volume are copied exactly to another volume, then a back-up volume of the same volume of the main volume is required, in order to store back-up data for a single generation. Consequently, if the back-up data is managed by means of a plurality of generations, then a large number of volumes are required and hence the back-up costs increase. Moreover, each time a new-generation volume is created, then it is necessary to execute an initial copy for transferring the whole volume, and hence the system load increases.
If the storage contents at a particular point in time are taken as a reference, and a differential snapshot is used to manage the differential only, then the required storage contents are relatively small compared to the aforementioned method, and back-up data can be managed in the form of a plurality of generations. However, the actual data used for managing generations on the basis of differential snapshots is not backed up, and there only exists one version of this data. Furthermore, the data for generational management cannot serve the purpose of data back-up on its own. The user is able to restore the data of a desired generation by referring to both the main volume forming the reference, and the data used for generational management.
Therefore, if a fault occurs in the main volume, it is not possible to restore data simply by means of the data for generational management in the differential snapshot. Furthermore, in the event that the data for generational management has been lost, it becomes impossible to revert to the storage contents of the main volume at a prescribed point of time in the past.